The present invention relates to a method and a device for determining residual stresses in objects, in particular in coated objects, and to a method and a device for coating objects.
Coatings are frequently used in order to ensure functional (e.g. in the case of corrosion or wear protection or in sensor technology) or decorative properties of an object surface. However, layers often have residual stresses caused by the production, which produce undesirable effects (e.g. layer spalling, crack formation). It is therefore of interest to know the residual stresses and their effects in the layer composite in order to carry out the layer production in a suitable manner.
A large number of techniques is known for determining residual stresses in layers and layer composite materials. Many methods, such as the use of beta emitters or the X-ray fluorescence technique, are mere laboratory measurement methods that are less suitable for industrial use. Techniques such as eddy current measurement methods, Barkhausen-Rauschen or inductive measurement methods can only be used for measurements on conductive or magnetic samples (cf. non-patent document [1]). Very high accuracies can be achieved with X-ray diffractometry. This method is based on the diffraction of X-ray radiation (determination of the Bragg angle), which is influenced by lattice distortions due to residual stresses. By X-ray diffractometry, residual stresses and the different proportions of the residual stresses of type I, II, and III (macro-, meso and microscopically) to the total residual stress state can be determined with a very high spatial resolution (cf. non-patent document [2]). However, these examinations are very time-consuming and therefore not suitable for measurements during a layer formation process.
Drilled hole or toroidal core methods can be used in a comparatively cost-effective manner. The drilled hole method (in the classic form or in the form of the micro-circular milling method) is a “minimally destructive” method that is often used in practice and represents the prior art in the practical application (cf. non-patent documents [3] to [5]). By drilling a hole, residual stresses are released, i.e. relaxed. This results in a resulting deformation (e.g. strain) of the surface. The measurement of the surface deformations (e.g. strains), in combination with suitable calibration functions (which have to be simulated for layer composites), allows the quantitative determination of the residual stresses. Residual stress depth profiles can be determined by incremental drilling or milling in small steps. The surface strains are traditionally measured using strain gauges (DMS). However, the use thereof is only possible on flat and relatively smooth surfaces. In addition to the practical disadvantage of the necessary direct application of the DMS to the test object, limitation to a measurement of two-dimensional (2D) deformations (lateral to the surface) is considered a further disadvantage. As in this case the measurement distance from the bore is always comparatively large, there are also limitations both with regard to the local resolution and to the practical applicability. Furthermore, these measurements are prone to errors with respect to asymmetries of the bore and the positioning of the DMS measuring grid rosette.
Instead of a bore, the residual stresses can also be released by local heating of the object surface (e.g. by a laser) (cf. non-patent document [6] and U.S. Pat. No. 5,920,017). If the material parameters and the heating or cooling cycles are known, the residual stresses can be inferred from the measured deformations.
Optical techniques allow a high-resolution areal detection of three-dimensional surface deformations and have therefore been used with various methods for determining residual stresses. Normally, these methods are based on classic holographic interferometry, speckle interferometry (electronic speckle pattern interferometry, ESPI) or digital holography. By evaluating the holograms/specklegrams, it is possible to determine the deformation of the surface after drilling. In connection with, for example, finite element simulations, the residual stresses can then be calculated (cf. non-patent document [5]). For measuring displacements, image correlation can be used as well (cf. non-patent document [7]).
The patent document AU 4147289A describes a camera-based holographic speckle interferometer, with which micro-deformations induced by stress (generation of mechanical tension) can be detected simultaneously and at different scales by combining different optical measurement methods. The degree of stress on the object is varied in this case.
The patent specification U.S. Pat. No. 5,339,152A discloses a movable interferometric arrangement, which is suitable for determining the residual stresses occurring in the event of a load varying over time in comparatively large, drilled holes for fastenings in a temporally resolved manner. The arrangement is particularly suitable for use in aircraft construction.
The patent specification JP 2004-170210 A describes a method for determining stresses by measuring the deformation of a drilled hole. In this case, a miniaturized laser distance sensor is used.
The patent specification U.S. Pat. No. 7,154,081 B1 describes an optical measuring system for the temporally resolved measurement of residual stresses on coatings, for example on insulator layers for electrical conductors. The optical measurement system comprises a plurality of spatially distributed optical fiber sensors, wherein Bragg gratings are used. A disadvantage of this measurement system is the low spatial resolution. Furthermore, it is not suitable for production processes having a high material throughput.
The non-patent document [8] describes a method for determining residual stresses at the micrometer scale, which is based on the measurement of deformations produced by incrementally drilling nano-holes (diameter 50 nm) using focused ion beam (FIB). The deformations are measured by a scanning electron microscope (SEM) using image correlation methods.
The patent application WO 2013-108208 (A1) describes another method for detecting residual stresses in surface coatings at the micrometer scale. To this end, two pairs of strips of the material are removed from the surface by electron or ion beam removal techniques. The lateral displacements resulting from the exposed residual stresses are measured by digital image correlation on the basis of SEM images (SEM: scanning electron microscope). However, the use of scanning electron microscopy does not allow a rapid measurement of the residual stresses of surface coatings in a production process having a high material throughput, since scanning electron microscopes usually operate in a vacuum.
Layers having residual stresses are partially produced by thermokinetic coating (“thermal spraying”). Thermokinetically deposited layers have inhomogeneities, porosities, and multi-phase structures, wherein the original molten spray particles partially form pronounced textured layers. They are anisotropic and therefore differ significantly from the corresponding solid materials in terms of their properties. Furthermore, with regard to their application properties (load-bearing capacity, reliability), they are strongly influenced by their residual stress states after layer application and finishing, which in turn depends on the material properties of the layer composite partners involved and on the process parameters.